1. Field of the Invention
The present invention relates to an apparatus for and a method of recording and/or reproducing information by illuminating a recording medium having a plurality of tracks with a light beam.
2. Related Background Art
It is known that recording media of the type which allow information to be recorded and reproduced by illumination of light have been provided in various forms such as discs, cards or tapes. In particular, there is a growing demand for card-like optical information recording media (hereinafter referred to as "optical cards") in the field of small-size, light-weight and transportable information recording media having large storage capacities.
FIG. 1 is a diagrammatic plan view showing a conventional optical card 101 of this kind. The card 101 comprises an information recording area 102, a plurality of information recording tracks 103, track selecting areas 104, 104' and a home position 105 for a light beam spot.
The optical card 101 is scanned by a light beam which is modulated in accordance with information to be recorded and which is formed into a miniature beam spot, whereby the information is recorded on the optical card 101 in the form of a recording pit array (information track which is optically detectable and hereinafter referred to simply as a "track"). In order to accurately record information without any trouble such as crosstalk between adjacent information tracks, it is necessary to control the illumination position of the light beam spot in the plane of the optical card in the direction perpendicular to the scan direction. (This control is hereinafter referred to as "autotracking or AT".) In addition, in order to illuminate the optical card with a stable miniature beam spot of light irrespective of the curvature and the mechanical error of the optical card, it is necessary to control the light beam in the direction perpendicular to the plane of the optical card. (This control is hereinafter referred to as "autofocus or AF".) During reproduction, the above AT and AF operations are required. The AF and AT operations are implemented, for example, by moving the objective lens incorporated in a recording and/or reproducing head (hereinafter referred to as an "optical head") in various directions.
A method of recording and reproducing information will be explained below with reference to FIG. 1. Initially, the light beam spot is located at the home position on the optical card 105. Then, the light beam spot is moved in the track selecting area 104 in the direction indicated by arrow u in FIG. 1 (in practice, an optical head which will be described later is moved), and searches for a track N to be recorded or reproduced. On the track N, the AT and the AF operations are started to scan the track N in the direction indicated by arrow r, thereby recording or reproducing information on or from the track N. In general, this scanning is performed by moving the optical card 105 in the track direction with the optical head held in position. When the light beam spot enters the track selecting area 104', only the objective lens is actuated with the optical head itself held in position, thereby moving the light beam spot to the adjacent information track (N+1). (This operation is generally called "kick".) Then, the light beam spot is made to scan the track (N+1) in the direction indicated by arrow l in FIG. 1 to record or reproduce information. Thereafter, in a similar manner, scanning of the light beam spot on the information tracks 103 and kicking of it in each of the track selecting areas 104 and 104' is repeated by the number of times corresponding to the amount of information.
FIG. 2A is a diagrammatic plan view showing the actuating section of an optical information recording/reproducing apparatus utilizing the aforesaid type of optical card, and FIG. 2B is a diagrammatic front elevational view of the same.
As shown in FIGS. 2A and 2B, an optical card 1 is securely mounted on a shuttle 2 which is fixed to a belt 3. The belt 3 is supported by pulleys 4a and 4b, and the pulley 4a is rotated by means of a DC motor 5. Accordingly, the motion of the motor 5 can be reversed to move the optical card 1 back and forth in the x axis direction.
An optical head 6 is disposed above the optical card 1 to movably hold an optical system 7. The optical system 7 is finely moved in the y-axis direction by means of an optical-system actuator (not shown), and a light beam 8 focused by the optical system 7 forms a light spot on the optical card 1 as will be described below. Accordingly, the light spot is finely moved in the y-axis direction by the optical-system actuator, thereby effecting the AT operation. Although the AF operation is also performed by the optical-system actuator, a description thereof is omitted.
A ball screw 9 is inserted through the optical head 6 in such a manner as to be rotated with a pulse motor 10. Accordingly, if a desired form of pulse is applied to the pulse motor 10 to control the direction and angle of rotation thereof, the optical head 6 is moved in the y-axis direction by the desired distance.
Needless to say, the AT operation is required to correct the offset between the objective lens in the recording and/or reproducing head and each of the tracks with respect to the direction perpendicular to the direction of the axis of each track, and one important cause leading to such positional offset is the offset between the direction of reciprocal motion of the optical card and the direction of the axis of each track. (This offset is hereinafter referred to as "skew".) As typical causes leading to such skew, it may be pointed out that a reference side 100 of the optical card 101 of FIG. 1 is not parallel to and is inclined with respect to the information tracks 103 and that the optical card is obliquely transported due to, for example, the imperfect operation of a card transporting mechanism. Incidentally, the skew quantity with respect to the position of the light beam spot in the track direction, that is, with respect to the amount of feed of the optical card, exhibits a linear variation, i.e., increases or decreases substantially proportionally to the amount of feed of the optical card.
The manner of recording or reproduction when skew takes place will be explained below with reference to FIG. 3.
In FIG. 3, the optical card 1 is provided with a recording area a, track jump areas b and c and tracking tracks T.sub.l to T.sub.n arranged at intervals of the distance yt.
In the recording area a, light spots S.sub.1 and S.sub.3 formed by the optical system 7 shown in FIG. 2B illuminate, for example, tracks T.sub.1 and T.sub.3, respectively, and light reflected therefrom is utilized to perform tracking control. A light spot S.sub.2 illuminates the data track defined between the tracks T.sub.1 and T.sub.2. During illumination, the power of the light beam 8 is increased to form a pit, while, during reproduction, the light beam 8 of low power illuminates a particular formed pit and the recorded information is reproduced from reflected light. In either of these operations, since tracking control is performed on the basis of the light spots S.sub.1 and S.sub.3, the light spot S.sub.2 consistently scans each data track along the predetermined axis thereof, whereby accurate recording or reproduction is enabled. .In FIG. 3, arrows shown adjacent to the respective light spots indicate that, when the optical card 1 is moved along the X axis in the negative direction thereof by the motion of the motor 5, the light spots relatively move along the X axis in the positive direction thereof.
Subsequently, when the light spots reach the track jump area b, the optical system 7 is finely moved along the Y axis in the positive direction thereof by an optical-system actuating means, whereby the light spots jump in the positive direction of the Y axis by the distance yt. Accordingly, the light spots S.sub.1 and S.sub.3 move onto the adjacent tracks T.sub.2 and T.sub.3. In synchronism with the above operation, the motor 5 is reversed to cause the light spots to relatively move in the negative direction of the X axis. Thus, recording or reproduction of the adjacent data tracks is started. Thereafter, each adjacent track is likewise scanned in sequence.
As explained above, a cooperation between the above track jump operation and the relative movement of the optical head 6 in the X-axis or Y-axis direction by the motor 5 and the pulse motor 10 enables the light spot to have access to the desired track in the recording area a, thereby enabling recording or reproduction of information.
However, in practice, because of a skew, the direction of movement of the optical card 1 is not parallel to the axes of the respective tracks T.sub.l and T.sub.n. For example, it may be assumed that the X axis shown by a dashed line represents the direction of movement of the optical card 1, that the Y axis shown by a dashed line represents the direction of movement of the optical head 6 and the optical system 7, and that .theta..sub.s represents the skew angle made between the X axis and the axis of each track T.sub.l to T.sub.n.
In the presence of the skew angle .theta..sub.s, if the light spot is relatively moved, the optical-system actuator means moves the optical system in the positive direction of the Y axis so as to cause the light spot to follow the track. However, the optical system 7 can only move within a particular range (.+-.Y max) and, if the range is exceeded, the light spot is deviated from the track and tracking control will be impossible. For this reason, to prevent the optical system 7 from exceeding the movable range, the optical head 6 is moved in the positive direction of the Y axis by means of the pulse motor 10 (hereinafter referred to as "head movement") before tracking control becomes impossible.
For example, in FIG. 3, the spot S.sub.1 sequentially scans the tracks T.sub.1 to T.sub.5 and, toward the middle of the recording area a, the optical system 7 reaches one boundary of the movable range Y max. At this position, the optical head 6 is moved in the positive direction of the Y axis by the pulse motor 10.
However, since the angular velocity of the pulse 10 is large, the acceleration of the optical head 6 in the Y-axis direction also increases. As a result, during the head movement, a certain degree of relative positional offset occurs between the light spot S.sub.1 and the track. If there are large dust particles or scratches on the optical card 1 or if an external vibration is transmitted to the optical card 1, a tracking error will easily occur. If the tracking error occurs in the recording area, the light spot will move to a neighboring recorded track, with the result that the recorded track is overwritten to damage the recorded information.
In addition, even if no tracking error occurs, since the light spot vibrates to some extent during a head movement, the data error rate of recording and/or reproduction may increase.